The present invention relates to a process for forming grooves on the outer diameters of a shaft of a spindle motor, wherein the grooves are used to generate appropriate hydrodynamic pressures in a hydrodynamic bearing.
Electric spindle motors of the type used in disk drives conventionally rely on ball bearings to support a rotary member, such as a rotating hub, on a stationary member, such as a shaft. Ball bearings are wear parts and in time friction will cause failure of the motor. In addition, ball bearings create debris in the form of dust or fine particles that can find their way into "clean" chambers housing the rotary magnetic disks which are driven by the motor. The mechanical friction inherent in ball bearings also generates heat and noise, both of which are undesirable in a disk drive motor.
Hydrodynamic or fluid bearings are often used as a replacement for ball bearings in disc drives and other apparatus having rotating parts. In a motor using a fluid bearing, the rotating member is separated from the stationary member by a film of lubricating fluid. A fluid bearing offers several advantages over ball bearings such as low non-repeatable run-out, low audible noise, and high damping. Accordingly, fluid bearings represent a considerable improvement over conventional ball bearings in spindle drive motors.
Examples of fluid bearings are shown in U.S. Pat. Nos. 5,427,456 and 5,536,088 and U.S. patent application Ser. No. 08/591,735 to Charles J. Cheever, et al., which have been assigned to applicant's assignee and are incorporated herein. The motors shown in these references generally include the following parts: a central shaft integral with and perpendicular to a horizontal thrust bearing plate; a hub assembly having a sleeve surrounding the shaft as well as a radial upper surface that supports the thrust bearing plate; and a flanged bearing ring supported by the thrust bearing plate. Although generally the shaft is stationery and the hub assembly rotates, such motors are easily adaptable to rotating shafts paired with stationery hub assemblies.
Fluid thrust bearings are formed by lubricant within fluid bearing cavities of the motor. For example, an interior fluid bearing cavity is formed between the shaft and hub assembly in which at least one fluid thrust bearing supports the hub assembly on the shaft. A groove pattern or spiral groove geometry is provided on the outer diameter of the shaft or the inner diameter of the sleeve to generate the appropriate fluid pressure gradient and hydrodynamic pressures to retain the fluid within the bearing cavity. Other fluid bearing cavities may be formed between parallel surfaces such as between the lower surface of the flanged bearing ring and the upper surface of the horizontal thrust bearing plate and between the lower surface of the horizontal thrust bearing plate and the radial upper surface of the hub assembly. One or more of these surfaces may have a spiral groove pattern to provide an inward pumping action that maintains a pressure differential, pushing fluid radially inward toward the shaft.
Spiral groove geometry generates hydrodynamic pressures which support rotating motor parts (such as the sleeve) relative to stationary motor parts (such as the shaft) resulting in a desired bearing stiffness. One procedure used currently for grooving hydrodynamic bearing surfaces is to machine cut the inner diameter of the rotating sleeve or the outer diameter of the shaft. Creating grooves in this manner limits the designer to using only soft bearing materials which wear prematurely. These processes also tend to create burrs on the surfaces.
Another method of creating the groove pattern, disclosed in U.S. Pat. No. 5,315,196 to Yoshida, et al. includes printing a resist on a shaft and then electroplating the shaft. When the resist is removed, a groove pattern remains. Yoshida, et al. also briefly disclose methods for chemical etching, rolling, and laser-forming (U.S. Pat. No. 5,198,637 to Noda, et al., also shows the use of a laser to cut grooves in a hydrodynamic thrust bearing surface). Regarding the rolling method, the Yoshida, et al. reference indicates that an unwanted "shoulder" is formed within the groove.
None of these previous process are satisfactory because they have one or more of the following problems: they provide little flexibility over the specific design of the grooves; they create burrs on the bearings surfaces; they preclude the use of hardened bearing surfaces; they form unwanted shoulders; they distort the shaft; or they are extremely expensive.